Enable RSpec/BeNil & RSpec/BeEq for Rubocop

[alecslupu]

🎩 What? Why?

This PR fixes the RSpec/BeNil and RSpec/BeEq rubocop violations revealed by #9201

📌 Related Issues

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• Related to Upgrade Rubocop libraries #9201

Testing

This PR does not add any Decidim functionality, it only fixes Rubocop issues

📋 Checklist

🚨 Please review the guidelines for contributing to this repository.

• ❓ CONSIDER adding a unit test if your PR resolves an issue.
• ✔️ DO check open PR's to avoid duplicates.
• ✔️ DO keep pull requests small so they can be easily reviewed.
• ✔️ DO build locally before pushing.
• ✔️ DO make sure tests pass.
• ✔️ DO make sure any new changes are documented in docs/.
• ✔️ DO add and modify seeds if necessary.
• ✔️ DO add CHANGELOG upgrade notes if required.
• ✔️ DO add to GraphQL API if there are new public fields.
• ✔️ DO add link to MetaDecidim if it's a new feature.
• ❌AVOID breaking the continuous integration build.
• ❌AVOID making significant changes to the overall architecture.

📷 Screenshots

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♥️ Thank you!

[alecslupu]

@ahukkanen thei can now be reviewed.

[ahukkanen]

Thanks @alecslupu !

LGTM, I adjusted the title of the PR to better describe what is done in this PR alone.